The purpose of the proposed research is to determine the quantitative relationship between neural activity in the cortex and intrinsic optical signals (signals that do not require the application of dyes). Optical imaging of the cortex, especially of intrinsic optical signals, has been used extensively in the past decade to explore the functional organization in the brain, especially in the visual cortex. Some of the reported results have been surprising and some remain controversial. Despite the natural appeal of the technique and the wide attention that its results have attracted, very little is known in quantitative detail about the coupling between the neural activity and the intrinsic optical signal, which has several components. Such knowledge is crucial for the proper interpretation of the activity and selectivity selectivity maps obtained with optical imaging. An investigation of the following questions is proposed: 1) What is the quantitative relationship between the neural activity and the optical signal? 2) Is the neural-optical relationship uniform across the cortex (isotropic)? 3) What aspects of neural activity (spikes, synaptic activity) are represented by the optical signal? 4) What is the contribution to the optical signal of the various functional or anatomical cell types in the cortex? The results of these studies will allow a quantitative interpretation of activity maps obtained not only by optical imaging, but also by other important imaging technologies that rely on the hemodynamic filter, such as fMRI and PET, which are now used not only in research but also as diagnostic tools in clinical practice.